The present invention relates to a method of adjusting the key touch of a keyboard and a device which carries out the method.
In order to minimize an operator's fatigue and improve efficiency when the operator handles a keyboard serving as an input unit for word processors or computer systems, keyboards having a comfortable key touch have been desired. Major factors which affect key touch, that is, the "key feel" with which the operator depresses key tops, are the magnitude of the stroke of a key top, the resistive force which the operator receives from the key top, and a click with which the operator knows that an electric input has been completed. Which key touch consisting of the combination of these factors is desirable depends on an individual operator.
In general, keyboards are constructed of:
(1) a plurality of switches, such as electrical contacts, which are opened and closed by depressing corresponding key tops; PA1 (2) a plurality of key tops for specifying the position of the plurality of switches on the keyboard and for transferring a depressing force to a selected switch; and PA1 (3) an electric circuit, such as an encoder or an interface, which transfers signals generated by opening and closing of the plurality of switches on the keyboard to a control unit, such as a computer.
Various types of switches can be employed depending an application or cost. Examples include a lead switch, a mechanical switch, a membrane switch in which two flexible films on which electrical contacts are formed in an opposed relation are laid on top of one another with a small gap therebetween, and a switch in which the films and contacts are replaced by a conductive rubber sheet.
FIGS. 1 and 2(a) and 2(b) are respectively a perspective view and a cross-sectional view of an example of a membrane switch which is most widely employed in a keyboard for a word processor, a personal computer or a terminal unit.
Referring first to FIG. 1, an upper film 101 made of, for example, polyester has a circuit pattern 101A and contacts 101C, while a lower film 102 has a circuit pattern 102A and contacts 102C. The circuit patterns and contacts are formed by printing using an ink which contains a silver powder. An ink with a carbon powder contained therein may also be printed on the surfaces of the contacts 101C and 102C in order to prevent electromigration of silver atoms. The films 101 and 102 are laid on top of one another with a spacer 103 in which holes are provided at positions corresponding to the contacts 101C and 102C provided therebetween.
Turning to FIG. 2(a) which is a cross-sectional view of a pair of contacts 101C and 102C formed on the films 101 and 102, respectively, and the surrounding area, in a state where no external depressing force is applied to the contact 101C, the contacts 101C and 102C are open due to the presence of the spacer 103, Application of a depressing force F to the contact 101 makes the film 101 curved and thereby brings the contact 101C into contact with the contact 102C, as shown in FIG. 2(b) . As a result, a current flows between the circuit patterns 101A and 102A, and depression of the key top (not shown) corresponding to the contacts 101C and 102C is detected.
FIG. 3 is a cross-sectional view of a key top 204 and elements which are associated with it. On a support panel 201 made of iron, aluminum or a plastic is disposed the membrane switch 200, which has been described with reference to FIGS. 1 and 2. A housing 202 is disposed on the membrane switch 200 in an opposed relation to the contact of the switch 200, and a slider 203 which moves by depression of the key top 204 is inserted into the housing 202. When the external force applied to the key top 204 is removed, the depressed key top 204 returns to a steady position by springs 205 and 206. Provision of two types of springs 205 and 206 allows the operator to have a desirable "key feel" when he or she depresses the key top.
When the key top 204 is depressed, the contacts (not shown) of the membrane switch 200 are closed by the spring 206, and thus selection of a predetermined key top 204 is detected. Detection requires an encoder or an interface to an external circuit. However, these are not related to the present invention, and description thereof is omitted.
To obtain a comfortable key touch, a stroke of the key top 204 of 3 to 4 mm is desired. Furthermore, to assure smooth movement of the slider 203 which is free from shaking or being caught, the length of the portion of the housing 202 into which the slider 203 is fitted must be 3 to 4 times that of the stroke, preferably 4 times that of the stroke.
FIGS. 4 and 5 are graphs of curves generally employed to represent key touch, i.e., key force profile curves which represent the relation between the depressing force applied to a key top and the displacement of the key top caused by it. The abscissa axis represents key top displacement, and the ordinate axis represents depressing force.
Referring to FIG. 4, as the operator depresses the key top with a finger, the key top begins to sink and a force proportional to the distance which the key top has sunk, i.e., a force proportional to the displacement of the key top, is applied to the finger. When the key top has sunk to a certain position, the force applied to the finger suddenly decreases. That is, the depressing force relative to the displacement decreases at that position. Normally, the contacts of the switch are closed at that position, and the operator senses by the "key feel" of sudden decrease in the force (a click) that key input has been done. When the key top is further depressed, the force proportional to the distance which the key top has sunk is applied again to the finger. When the depressing force is further increased, the key top reaches the position where it cannot be displaced any more. The total displacement to that position is the stroke of the key top. The inclination of the curves shown in FIG. 4 is determined by, for example, the spring constants of the springs 205 and 206 in the structure shown in FIG. 3. To impart a change of decrease in the depressing force, as shown in FIG. 4, a spring 206 may be employed which yields at the depressing force applied immediately before decrease in the depressing force occurs.
FIG. 5 is a graph showing a key force profile curve which exhibits hysteresis. The key force profile curve shown in FIG. 5 is employed more extensively than the curve shown in FIG. 4.
The curve shown in FIG. 5 exhibits step increase and hysteresis characteristics. The step increase in depressing force eliminate shaking of the key top, which would occur at the initial stage of depression, and to prevent displacement of the key top when the depressing force is lower than a fixed value. The hysteresis enables chattering to be suppressed by differing the positions of the key top, corresponding to closing and opening of the switch.
That is, in the depressing process, the contacts of the switch are closed when the key top has been displaced to a position indicated by `b` on the abscissa axis. In the returning process, the contacts of the switch are opened when the key top has passed the position indicated by `b` and returned to a position indicated by `a`. At position `b` the force applied to the finger suddenly decreases, while at position `a` the force applied to the finger suddenly increases. Thus, in the depressing process, even when the key top slightly chatters in the vicinity of the position `b`, after it has passed the position `b`, the closed contacts do not open unless the key top returns to the position `a`, and chattering of the contacts can thus be prevented.
Which pattern of the relation between the displacement and the force applied to the finger, i.e., which key touch, among those represented by the key force profile curves is desired depends on an individual operator. Some operators prefer relatively hard key touch (a large spring strength) and other operators like soft key touch (a small spring strength). There are those who feel the "key feel" of sudden change in the depressing force annoying. Thus, when key touch is evaluated, click must be taken into consideration in addition to the stroke of the key top and the magnitude of the force applied to the finger.
However, in a conventional keyboard, the shape of the key force profile curve is determined by, for example, the structure of the slider 203 shown in FIG. 3 and the characteristics of the two springs 205 and 206, and it is thus impossible to adjust key touch according to the liking of an operator. For the operator who does not like the key touch of a given keyboard, there is no remedy but to get used to it. This is very unpleasant, and is undesirable in terms of fatigue and inefficiency which derive from use for a long time.
When design of a keyboard is determined conventionally, a plurality of keyboards having, for example, different strokes and spring strengths are prepared, and the key touch of the product is determined by adding up the results of the evaluations made by a plurality of test operators. Assuming that the test operators preferred spring strengths of 40 grams and 60 grams among the five types of spring strengths from 20 grams to 100 grams which are each different from the previous one by 20 grams, ten types of test keyboards, which are combinations of five types of strokes from 1 mm to 5 mm which are each different from the previous one by 1 mm and two types of spring strengths, 40 grams and 60 grams, are prepared for evaluation. Thus, whereas enormous cost and time are required to manufacture a plurality of types of test keyboards, the results of evaluations made on only several tens of samples are obtained. Furthermore, the key force profile curve representing the relation between the depressing force and the displacement of the key top is determined only by the optimum stroke and spring strength obtained in the manner described above. Thus, evaluations are made only on several key force profile curves whose positions where click occurs differ from each other, i.e., whose hysteresis characteristics differ from each other, and selection is made from only two or three types of keyboards.